1. Field of the Invention
The present invention relates to the manufacture of plastics containers and is particularly applicable to the manufacture of open top vessels from thermoplastics materials which are suitable for use in food containers.
The forming of shaped articles such as shallow trays, cups and open top vessels from a sheet of thermoplastics material is well known. Techniques such as vacuum forming or thermo forming are widely practised. With the advent of thermoplastics laminates or co-extrusions having at least one layer of a material such as polyvinylidene chloride (PVDC) or ethylene vinyl alcohol (EVOH) providing a barrier to the migration of oxygen through the walls of plastics vessels, thermo formed containers have been increasingly adopted for the packaging of foodstuffs.
A thermo formed container for foodstuffs typically comprises an open topped vessel or pot and a removable closure. Although the wall of the pot may be formed from a material having an oxygen barrier layer to retard spoilage of the contents of the container by oxidation, it is also important that the closure comprises an oxygen barrier, and that the join between closure and pot forms a complete seal whereby to fully seal the contents of the container if foodstuffs or other items subject to spoilage are to be stored in the container.
One preferred closure comprises a metal closure of the ring pull type adapted to be crimped or otherwise secured about an outwardly extending circumferential flange around the open top of the pot. The closure typically has a ring pull tab and a weakened zone and/or one or more score lines suitable to facilitate opening of the container such as by removal of the top from the pot.
The integrity of the seal between the pot and the closure is an important element in the preservation of the contents of the container. It will be appreciated that any ingress of oxygen through a faulty or misaligned closure may result in spoilage of the contents.
The pot for such a thermo formed container is typically formed by taking a band of hot thermoplastics material and passing it to a thermo former where a plurality of pots is formed in the band and upon cooling the pots are cut from the band whereby to provide a plurality of pots and a remaining web which can be recycled.
Unless the band of thermoplastics material is very accurately located at the cutting knives, it has been found that the outwardly extending flange may not be symmetrical once the pot is separated from the web. Defects in the flange such as those caused by misalignment of the web with the knives may result in an imperfect seal. Such an imperfect seal can result in the ingress of oxygen and spoilage of the contents when the container is used for carrying food.
2. Discussion of the Prior Art.
One proposal for forming a more symmetrical flange has been the trim in place technique in which plastics articles are separated from the surrounding web within the moulding die. Examples of the trim in place technique are shown in U.S. Pat. Nos. 2,522,956 (Middleton), 3,173,174 (Edwards) and 4,755,129 (Baker et al).
The trim in place technique however has problems relating to excessive wear of cutting edges operating at elevated temperatures and the need to suspend moulding operations when replacing cutting edges. In addition uneven contraction during cooling can result in pots formed by trim in place techniques being unsuitable to receive metal closures.
U.S. Pat. Nos. 4,308,005 (Zundel) and 4,442,064 (Myers et al) both describe an apparatus in which the pots are cut from the web in a separate step. In Zundel the conveyor of the cutting press is drivingly connected with the conveyor of the thermoforming machine. However, the strip steel cutters disclosed are unsuitable for providing a flange sufficiently symmetrical to receive a metal closure.
Myers et al relates to the precise trimming of thermoformed articles from a plastics web. The disclosure teaches the loose aligning of formed articles with article punches, isolating the articles from each other so that they exactly register themselves with the punches in a selected sequence and precisely trimming the pots from the plastic web. However, to achieve this Myers et al teaches initially shearing the web in the machine direction before separately indexing each of a plurality of strips containing one row of thermoformed articles. Each article is then separated from all following articles in the remainder of the strip by an across machine shearing operation.
However, the present invention by using continuous cutting surfaces simplifies the handling steps by avoiding separate separation in the machine and across-machine directions and is suitable for the efficient cutting and trimming of a number of rows of pots simultaneously.